Magnetic article surveillance system and method

Details Magnetic article surveillance system and method Magnetic article surveillance system and method

1988-01-28

1988-12-13

Goldstein, Herbert

Communications: electrical

Condition responsive indicating system

Specific condition

340551, 340568, 342442, G08B 1318

Patent

active

047914122

ABSTRACT:
A surveillance system based upon generation and detection of phase shifted harmonic signals from markers on articles or persons, responsive to transmission of a reference signal at a fundamental frequency in a detection zone, may be adversely affected by the presence in the detection zone of large metal objects, such as shopping carts. Such objects can alter the phase of the harmonic signals from predetermined values. An alternate signal processing technique for such a surveillance system recognizes that immunity to such disturbances can be achieved by measuring the phase between two or more of the harmonic signals returned from the marker, because the effect of such large metal objects has been found to have the same effect on all of the harmonic signals. Correspondence to a predetermined phase difference is a reliable indicator of marker detection.

REFERENCES:
patent: 4249167 (1981-02-01), Purenton et al.
patent: 4622542 (1986-11-01), Weaver

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